Pros and cons of replacing tool batteries with Lithium Polymer

[HammyDude] was tired of buying replacement batteries for his power tools. He had some Lithium Polymer batteries on hand and decided to take one of his dead drills and swap out the dead power pack.

The orange battery pack you see above has a deans connector on it for use with RC vehicles. By opening up the drill housing, [HammyDude] was able to add the mating deans connector. Now the replacement easily plugs into the drill, and it even fits inside the handle body.

This battery is made up of several cells, and an inexpensive charger is capable of topping off each individually for a balanced charge. In the video after the break [HammyDude] points out that the Makita charger applies voltage to all of the cells in series. It’s incapable of balance charging so when one cell dies the battery is toast. We’ve encountered this problem with Makita tools before.

One drawback to take note of in the end of the video: this replacement doesn’t have any low voltage cut-off. Running this battery pack down too low will permanently damage it. There must be a simple circuit that could be added as a safety measure. If you know of one, drop us a tip.

RC LiPos don’t have cutoff circuits for good reason. It is bad news if your battery suddenly turns off in-flight. Most small to mid sized RC aircraft have battery cuttoffs built-in to the motor control/regulator circuit. These have ways of warning the pilot on the ground that the battery is almost dead – like cutting power to the motor while leaving power available to the flight control surfaces. Other more advanced TX/RX units are doing telemetry now as well, so that an alarm triggers on the transmitter when the receiver senses low voltage.

For people who use Li-Ion powered tools and such and also fly RC with Li-Ion batteries, it might be worth investigating whether you can use your RC charger for your power tool batteries, especially if the Makita charger does not do balance charging.

Sure, you’d have to rig up some custom adapters and such, but it seems like it’d be very worth the hacking to be able to charge faster and increase the life of the battery.

I found a old 9.6v Makita at Goodwill this summer and did the same hack. A 3cell 11.1v lipo will fit in the handle and the extra volts will give it a nice boost. I used a small low volt buzzer ,also in the handle , to solve the problem of over discharging. It’s a great hack.

I use a MAX6457 (http://www.maxim-ic.com/datasheet/index.mvp/id/3492) for the low voltage cutoff. The cutoff voltage is set with a couple of resistors, and the output can be used to control a MOSFET to disconnect the battery as voltage drops below the threshold.

Wire a series connected zener+LED+resistor (resistor sized to give say 10mA through the chain so the LED lights and the zener does its thing)with a zener voltage = battery end voltage – 1.6V – resistor voltage drop. Wire it across the motor or device load. When you use the device (drill, whatever), the LED will light brightly when the battery has enough volts. When it goes very dim, it’s close to the end-voltage and it’s time to charge that sucker.

You need a n-channel power MOSFET with low rDSon, a resistor to bleed the Vgs down when you want it off, and a zener diode to bias the gate when battery voltage is above the limit.

Negative lead (current return) connecting the battery pack with load is cut and MOSFET source is connected toward “-” battery terminal, while the drain is connected toward the load.

Thus, MOSFET works as low-side switch for the motor/load.

Zener diode cathode, the “sense” terminal of the cicrcuit, is connected to the battery “+” terminal. When battery voltage is higher then Vz + Vgson, the MOSFET D-S will conduct. When battery voltage is lower then that threshold, MOSFET will be shut.

You see, my first sketch was with circuitry on the side of the motor, but then I thought “this would get too complicated to present in purely textual description when direction switch is added” (or is direction switch a mechanical device on drill’s gearbox?) so I decided to describe it as if it was on the battery side.

I have recently worked on a high power LiFePO4 single-cell design. As says above, you need a FET (or two in parallel to reduce Rdson) in series with the battery output. If you can’t find a battery pack safety circuit ready-made for your pack’s cell count and max output current, you can probably design one with some 8-pin SOIC FETs and a Seiko controller like the Seiko S82xx series.

You can save yourself some money by eliminating the overcharge protection FETs if you have a good charger and will be charging the pack externally.

When I read “Pros and cons of replacing tool batteries with Lithium Polymer” my immediate thought was that someone was not only going to explain this nickel to lithium base swap… but also explain the pros and cons.

There are many pros and cons people are unaware of. Got my hopes all up. :)

One warning: do not assume that, just because LiPo cells can deliver much more current than, say, a NiMH pack, they will. There are potential limitations in the control circuitry and the tools’ motor; it might be that the original battery wasn’t actually the bottleneck. In that case, a LiPo powered tool will work perfectly, but if it was gutless before, it’ll remain so afterwards.

Thanks for the comments everyone. We did this in approx 15minutes, and, as indicated it is primitive with no engineering thought at all. As we mention in the YouTube comments, there is no discharge rating on this cheap LiPo cell, however, in our experience they can put out extraordinary amounts of power.* At the moment, current limiting is not high on my list of priorities. However, low voltage protection is something I’ve done a little bit of investigation into and I appreciate the links and commentary here. My goal is to be able to whip up a very simple PCB and have some protection.

As for the comment about adding balancing to the Makita pack and using a separate charger… we’ve thought about it. We don’t have 5cell chargers handy so it is something we’ll look at.

thanks
*mike

* We have shorted mostly dead cells and they results are impressive, these are scary powerful/dangerous.

you can make a little voltage measure circuitry wich would take measures from individual cells and if one of the cell’s voltage drops off, a buzzer would scream for you to tell that is time to recharge. so the alarm would be audible an visual

These drills can take a peak amperage of around 15Amps or more so the protection circuits need to be rated for at least that. I did a similar swap and tried a 4 cell configuration with 7A protection circuit from ebay and it did not work. 7A was just barely enough for the smallest screws. Also the lithium battery needs to be rated for that kind of current. E.g. scavenged 18650 cells from laptop batteries are great for many projects but they don’t work for this kind of load. Finally I ended up using a 3 cell RC battery which can give up to 45A continuous current. The ability to deliver amperage really matters.

there are drawbacks that can be overcome by packing the battery in a way to decrease vibration. Lithium polymer tends to swell when drained at high amperages or when damaged by shock. the price of the batteries has dropped so swaping out the old nickel cadmium cells over to lithium polymer is economical, not to mention the advantages that come with using lithium batteries.Can’t wait till lithium sulfur comes out.